I currently have a 100amp main service box but it is all full and some are even doubled up. I'm wondering if I can hook up a 100amp subpanel off of it. If so what size wire to go with?

It would be on the same wall next to it within 12 inches. Do I have to use a conduit between the two, or does it have to exit the bottom of the main to the top of the subpanel?

Also, all I'm seeing at a local store are 100 amp main boxes. Is that allowable by code to use as a sub? I know the sub can't have neutral bonded by that screw either way.

My local town in Illinois uses the 2011 NEC handbook. Sorry for so many questions, but I just want to do it right. Thanks.

  • 8
    Hey, you may knows this, but just a heads up: The mains (the big wires from outside) coming into your main panel are (probably) always hot, even if you turn the main breaker off at the top of the panel. The main breaker turns off power to everything downstream, but the wires before it are still hot. Usually, there's not much exposed, but the screws to the lugs are always exposed so don't touch those, don't even get close. Both screws are live and want to kill you and your tools. Commented Jan 16, 2018 at 15:29
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    As someone who had there panel upgraded and it almost full, again, get an electrician and replace the panel. Get 300A,and get one with 60-80 breaker slot, so you don't run out. Honestly I would buy one with 100 breakers if I could.
    – cybernard
    Commented Jan 16, 2018 at 22:45
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    300A and 100 breakers - what the heck are you running @cybernard??
    – cr0
    Commented Jan 17, 2018 at 16:38
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    @cr0 Honestly nothing special. We remodeled, and its like this, oh by the way 201# code book requires separate circuits for <a long list of items> We have 40 full, and only 1 working outlet upstairs. Appliance outlet has to be dedicated. Fridge,dishwasher,microwave, lights, 2x regular outlets and that is just the kitchen. 2x exterior outlets separate for mower,snowblower. Exterior lighting a couple more. Each room has smoke detectors at least 1 circuit. 3 for computer room. I think bedrooms have 2ea. Plus many for outlets and lights in the basement. Many of them separate for power tools.
    – cybernard
    Commented Jan 17, 2018 at 22:25
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    @cr0 My only comfort is I had 2 panels install, one for upstairs and one for down, and upstairs is still almost empty. drive.google.com/open?id=1ZpLp53PKExojWXlNZSU3ynH1m9mBxW1A6w There are number of things I wish I had room for, and most has to be eventually duplicated for upstairs.
    – cybernard
    Commented Jan 18, 2018 at 3:29

4 Answers 4


Let me try and answer some of your questions. First the panels you are looking at that are rated 100A simply means you can use them for any application up to 100A. You can for example add a 60A breaker to your existing panel and protect the new subpanel with a 100A rating. It is not so much what the panel is rated as what the protection is rated.

You can use other methods other than conduit but first you need to select what type of method you prefer to use and that must be permitted as outlined in Chapter 300 of the NEC "Wiring Methods". So you need to select a method then follow the installation directions as outlined in that particular selection. As an example, let's say you want to use type NM (romex) to connect the new sub panel. That would be Article 334 in the NEC. It will have uses permitted with exceptions and uses not permitted and give installation rules you must follow depending on where these panels are and the type of environment they are in.

I would like to suggest that it may be a better idea to replace and upgrade the existing panel. I believe this would be a better method and give you a better value added rather than several subfed panels.

  • Tester101 and myself were answering the same question at about the same time. Commented Jan 16, 2018 at 14:08
  • great info for me to take in. I should have been more specefic on what was going to be hooked up to subpanel. I was going to hook up my basement lights,bathroom,and receptacles. I have them all on separate runs and wanted them to have tgeir own box.So maybe the 100 amp subpanel may be to much.I agree a sercice upgrade would be better if i can afford it.Would they have to change the service cable that runs from pole to meter that runs underground?my environment would be in a basement and current panel is secured to the cement wall Commented Jan 16, 2018 at 15:04
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    @tracyrobert Even if you put a double pole 60 ampere breaker in the main panel to protect the new panel feeder, you can still install a 100 ampere panel as the second panel. As Retired Master Electrician points out, the panel rating is the maximum the panel can be used for, not what it has to be used for. As for the service upgrade... If the existing service conductors are not large enough, they would have to be replaced if the service was upgraded beyond their capacity. You'd have to check with the local utility, to determine who pays for that.
    – Tester101
    Commented Jan 16, 2018 at 15:14

If your panel is full, you'll likely want to have the service evaluated, to determine if it's still large enough to meet your needs. It's possible that you may want to upgrade to a larger service, which will likely require new service conductors and a new service panel. If this is the case, you'll simply have a larger panel installed.

If you don't have/want to upgrade the service, you could still have a larger panel installed. Just make sure the new panel is larger enough for any future expansion.

If you do want to install a second panel, you'll have to determine what size feeder you want to supply it. This will dictate the wire size, and the size of the breaker supplying the new panel. To figure out what size feeder you'll want, you have to decide which circuits you'll move over to the new panel.

Once you've got that worked out, you'll have to reroute all the selected circuits to the new panel location. This could require making junctions in the old panel, to extend the length of the circuit.

All in all, I'd say this is not a project for a novice DIYer. There are a lot of subtle details that will likely be overlooked by a non-professional (and maybe even some "professionals"). My advice, would be to get quotes from a few local licensed electricians. Make sure to get pricing for the options I described above, and then decide how to proceed.


  • I am in the same predicament as OP with only 2 spaces left in my 20 space panel that is on the outside of the house. I would really like to install a subpanel inside the house and move all inside circuits to it and keep all outside circuits (lamps, shed, external outlets etc.) in the main panel. I have read that a subpanel can't exceed 80% of the amperage of the main panel. Is that an old wives' tale or is there really a code to that effect?
    – cryptic0
    Commented Dec 5, 2020 at 0:56

I currently have a 100amp main service box but it is all full and some are even doubled up.I'm wondering if i can hook up a 100amp subpanel off of it?

Certainly, that is what sub-panels are for.

If so what size wire to go with?

The wire should be #3 copper for 100 amps and a 100 amp two pole breaker to feed it.

Do I have to use conduit between the two, or does it have to exit the bottom of main to top if subpanel?

No, and no. You choose where to install it and what wiring method to employ. It just cannot be higher than 6 1/2' to the top of the highest breaker.

Also all I'm seeing at local store is 100 amp main boxes. Is that allowable by code to use as a sub?

Yes, just don't use the bonding screw or strap.


What you want in the subpanel is enough spaces.

Get a subpanel that is large enough (in amps) to give you enough spaces to future-proof your house. Keeping in mind a few things:

  • the way you are using double-stuff breakers right now, is not a viable long-term strategy. New work now requires AFCI or GFCI breakers, and those are not available in double-stuff, each circuit will need a full space.
  • having spaces lets you do stuff you wouldn't otherwise do.
  • Plug-in electrics (including engine-too types) are exploding in the auto industry (just go to The North American Auto Show or view their app) and we foresee that mixing with the smart grid too (your car backfeeds to help the grid stay up, and you get paid for that) - that could well be coming to your garage. So at least 2 more breaker spaces.
  • On-demand hot water is making more and more sense, 2-8 spaces depending on setup.
  • it may be a good long-term strategy to eventually make this panel the main panel.
  • Some people want generators, off-grid switchable solar, or grid-tied solar. More spaces.

So it isn't wrong to buy a 42-space 225A subpanel. You can feed that from any breaker <=225A, including 150A, 100A, 60A, 30A and even 15A. The key is to think ahead to give yourself freedom to maneuver, rather than painting yourself into a corner.

  • I am saving this answer. Terrific information.
    – cryptic0
    Commented Jul 4, 2021 at 3:21

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